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Editors Selection IGR 8-1

Examination methods: From OHT to glaucoma

Comment by Balwantray Chauhan on:

14062 Optic disc and visual field progression in ocular hypertensive subjects: detection rates, specificity, and agreement, Strouthidis NG; Scott A; Peter NM et al., Investigative Ophthalmology and Visual Science, 2006; 47: 2904-2910

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It is often thought that structural damage in the optic disc precedes visual field damage in ocular hypertension and glaucoma. The evidence for this view is primarily based on clinical observation, while studies that have addressed issues of progression criteria, specificity and measurement noise are rather sparse in the published literature. In this study, Strouthidis et al. (792) studied visual field (with standard automated perimetry) and optic disc changes (with scanning laser tomography) measured longitudinally in a group of 198 ocular hypertensives and 21 control subjects. Using a variety of change criteria based on linear regression, the authors concluded that depending on the specificity of the criteria used, between 27.2% (specificity = 97%) and 53.5% (specificity = 90%) of ocular hypertensives showed either visual field or optic disc progression over an average of 6 years.

However, there was relatively few patients who showed both field and disc changes. While these progression rates appear to be high compared to results of the Ocular Hypertension Treatment Study, and are more in line with progression rates in patients with early to moderate glaucoma (with similar specificity rates), they agree with earlier findings that there are surprisingly few glaucoma patients who show both field and disc changes. The ocular hypertensives in the study of Strouthidis and colleagues had more visual field examinations compared to optic disc examinations and when the number of field and disc examinations were equalized, twice as many patients has optic disc compared to visual field changes. This evidence may suggest a higher chance of detecting change with more examinations.

Between 27% and 53% of OHT showed either VF or OD progression over 6 years

Criteria for progression are based on statistical and not biological events. As our clinical measures of progression are only surrogates (perhaps poor ones!) of what we actually want to measure, namely the structural and functional change at the retinal ganglion cell level, it is not surprising that carefully performed studies such as that of Strouthidis et al show poor correlation between structure and function. We need to fully investigate the impact of measurement variability, length and frequency of follow-up and a variety of perhaps unknown factors on the detection of glaucomatous change. In the meantime and based on the typical length of a scientific study which is undoubtedly a fraction of lifetime glaucoma, these authors and others have sensibly suggested the importance of both functional and structural tests in the monitoring of glaucoma.

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